1. Field of the Invention
The present invention relates to aluminum heat exchangers of a serpentine type having a serpentine-anfractuous flat aluminum metal tube and a plurality of corrugated aluminum metal fin units, and in particular, to such heat exchangers adaptable for refrigerant evaporators in an air conditioning system.
2. Description of the Prior Art
Heat exchangers of the serpentine type have been used for, for example, a refrigerant evaporator in an automotive air conditioning system, as shown in, for example, U.S. Pat. Nos. 4,350,025 and 4,353,224.
The serpentine-type heat exchanger comprises a flat metal tube having a refrigerant passageway or parallel passageways therein extending in a longitudinal direction of the tube. The flat tube is bent to weave up and down, or formed in a serpentine-anfractuous shape, and therefore, has a plurality of parallel portions spaced apart from one another and a plurality of U-shaped curved portions connecting adjacent ones of the parallel portions, respectively. A plurality of corrugated fin units are disposed in spaces between adjacent ones of the parallel portion of the tube and are joined thereto by brazing. Each of the corrugated fin units is formed by bending a thin plate in a corrugated form so that a number of crests are formed in opposite side surfaces of the unit alternatively. The crests in the opposite sides of the unit are joined by brazing to flat side surfaces of the opposite parallel portions of the tube.
As high heat-conductivity materials for the flat tube and the fin units, aluminum metals including aluminum and aluminum alloy are usually used. Such heat exchangers using aluminum metals are referred to as aluminum heat exchanger.
In a known serpentine-type aluminum heat exchanger, the serpentine-anfractuous flat tube is usually made of an aluminum metal having 99 wt. % or more Al , for example, AA 1050 (which comprises, by weight, 0.25% or less Si, 0.40% or less Fe, 0.05% or less Cu, 0.05% or less Mn, 0.05% or less Mg, 0.05% or less Zn, 0.03% or less Ti and 99.50% or more Al). While, an aluminum alloy brazing sheet is used for preparing the corrugated fin unit member, which has a core metal of AA 3003 (which comprises, by weight, 0.6% or less Si, 0.7% or less Fe, 0.05-0.20% Cu, 1.0-0.5% Mn, 0.10% or less Zn and the balance Al) with a cladding of an aluminum alloy brazing filler metal, such as AA 4343, 4045 or 4047 (which comprises, by weight, 0.30% or less Cu, 5-13% Si, 0.8% or less Fe, 0.15% or less Mn, up to 0.1%Mg, 0.20% or less Zn, up to 0.20% Ti, and the balance substantially Al). The brazing sheet is formed in a form of the corrugated fin unit, and the fin unit members thus formed are disposed in spaces between adjacent ones of parallel portions of the flat tube so that the crests in the opposite sides of each fin unit member are in contact with the opposite parallel portions of the flat tube. Then, the flat tube and fin unit members are heated in the assembled relation to a brazing temperature of about 600.degree. C., and are joined by brazing.
In the known serpentine-type aluminum heat exchanger, the flat tube tends to suffer from pittings by corrosion because the aluminum alloy AA 1050 of the flat tube is baser in the corrosion potential than the aluminum alloy AA 3003 of the fin unit material. However,useof another aluminum metal having a corrosion potential equal to, or baser than, that of the flat tube for the core metal of the brazing sheet results in deformation of the fin units during the brazing operation, because elements of the aluminum alloy brazing filler metal diffuse into the core alloy during the brazing operation to lower the melting point of the core metal. Further, the core metal becomes nobler than the flat tube as another result of the diffusion, so that the flat tube still tends to suffer from the pittings.
Moreover, the use of the brazing sheet results in high cost of the heat exchanger.
Furthermore, in the known serpentine-type aluminum heat exchanger, the fin unit has a coating of the aluminum alloy brazing metal layer which is lower in the heat conductivity than the core metal and the flat tube. This means that the aluminum alloy brazing metal layer on the fin unit degrades the heat exchanging property of the exchanger.